The type of GTO thyristor that this invention is concerned with comprises a semiconductor body having at least four layers, with contiguous layers being of different P and N conductivity types and with one end layer constituting an anode layer, the opposed end layer constituting a cathode layer (or emitter), and an intermediate layer contiguous with the cathode layer constituting a gate layer, which has a gate electrode in ohmic contact therewith. When the thyristor is in its "on" state, power current enters the anode layer through an anode electrode thereon and flows through the semiconductor body between the anode and cathode layers, exiting through a cathode electrode.
Turn-off of this GTO thyristor is effected by forcing a turn-off current between the cathode electrode and the gate electrode via a path that extends first through the PN junction between the cathode layer and the gate layer (referred to hereinafter as junction J1) and then laterally of the gate layer into the gate electrode. This turn-off current flows through junction J1 in a direction opposite to the power current flowing therethrough.
To enable high values of anode-to-cathode current to be turned off by the GTO thyristor in response to a gate turn-off signal of the above character, the cathode layer of the thyristor is divided into many elongated fingers that share the anode-to-cathode current through the thyristor. In ohmic contact with each of these cathode-layer (or emitter) fingers is a registering portion of the cathode electrode. The gate layer has predetermined surface regions immediately adjacent the cathode-layer fingers that are not covered by these fingers and extend alongside the junction J1 between the associated cathode-layer finger and the gate layer. The gate electrode is in ohmic contact with these predetermined surface regions of the gate layer and has portions that extend alongside the cathode-layer fingers, but in spaced relationship to the associated fingers and cathode electrode portions thereon.
The above-described turn-off current is shared by all the cathode layer fingers, flowing through the associated PN junction J1 adjacent each finger, then laterally of the contiguous gate layer into the associated gate electrode. Each cathode-layer finger acts as the cathode layer of a miniature GTO thyristor, the miniature GTO thyristors all being connected in parallel and sharing the anode-to-cathode current through the overall thyristor. In a successful turn-off operation, the turn-off current, shared by all the miniature thyristors, forces to zero (or squeezes off) the anode-to-cathode current through all of them, though not precisely at the same instant. During the brief interval immediately after such current squeeze-off, all of the miniature thyristors, including the last one to have conducted, must withstand the voltage imposed across the J1 junction of the device without such a resumption of current thereacross that would again trigger any of the miniature thyristors into conduction. Such a retriggering would cause large values of anode-to-cathode current to flow through the retriggered miniature thyristor, and this would cause a failure of the overall thyristor.